The present invention relates to a microfabrication method and apparatus, and more specifically, relates to a method for separating and extracting, using an ion beam, a micro-sample from a semiconductor wafer, a semiconductor device chip or the like, which includes a specific micro-area thereof, thereby to prepare a sample for carrying out observation, analysis and measurement about the foregoing specific micro-area, and further relates to an apparatus for implementing such a method.
In recent years, size reduction of semiconductor elements has been rapidly developed, and structural analyses of those semiconductor elements have been requiring observation of microstructures that can no longer be achieved by resolution of a normal scanning electron microscope (hereinafter referred to as “SEM”), and hence, observation based on a transmission electron microscope (hereinafter referred to as “TEM”), instead of the SEM, has been becoming essential. In this TEM observation, it is necessary to process an observation object to have a film thickness through which an electron beam can be transmitted, for example, a thickness of about 100 nm. As a method of preparing such a TEM sample, there is available a method wherein, using focused ion beam (hereinafter referred to as “FIB”) processing, only a portion, to be observed, of a sample substrate is extracted as a micro-sample by the use of a probe. This method is disclosed in International Patent Publication No. WO99/05506 (known example 1). First, marking is applied to an observation area (membrane forming area for TEM observation) on a sample substrate using FIB processing or the like. Then, two rectangular holes 202, 202′ are formed by irradiation of an FIB 201 on extensions of a straight line connecting between two marks 200, 200′ and on the outer sides of the respective marks 200, 200′ (FIG. 2(a)). Then, an elongate vertical trench 203 is formed by FIB scanning such that the vertical trench 203 extends in parallel with the straight line connecting between the marks 200, 200′ and has one end reaching the rectangular hole 202 and the other end slightly not reaching the rectangular hole 202′. A residual area 204 left between the rectangular hole 202′ and the vertical trench 203 will serve as a support portion for temporarily retaining a micro-sample including the foregoing observation area when separating the micro-sample from the sample substrate later (FIG. 2(b)). After inclining the surface of the sample substrate that has been held horizontal in the foregoing steps, an inclined trench 205 is formed by FIB irradiation in parallel with the straight line connecting between the marks 200, 200′ and on the opposite side of the straight line relative to the previously formed vertical trench 203. Here, since the straight line connecting between the marks 200, 200′ is set parallel with an inclined axis of a specimen stage (not shown), the surface of the sample substrate is inclined such that the side of the inclined trench 205 is raised relative to the side of the vertical trench 203. The inclined trench 205 is formed so as to connect between both rectangular holes 202, 202′. The inclined trench 205 at its bottom joins the bottom of the previously formed vertical trench 203. As a result, a part of micro-sample 206 of a wedge shape including the marks 200, 200′ is separated from the sample substrate, leaving only the residual area 204, so as to be cantilevered by the residual area 204 (FIG. 2(c)). Then, after restoring the surface of the sample substrate to be horizontal, a tip portion of a probe 207 of a sample transfer apparatus is brought into contact with an end portion of the part of micro-sample 206 opposite to the residual area 204. Then, for fixedly connecting the tip portion of the probe 207 to the part of micro-sample 206, an FIB 201 is irradiated (scanned) on an area including the tip portion of the probe 207 while supplying deposition gas, thereby to form a deposition film 208 on the FIB irradiated area. The tip portion of the probe 207 and the part of micro-sample 206 are fixedly connected to each other via the deposition film 208 (FIG. 2(d)). For extracting the part of micro-sample 206 from the sample substrate, the residual area 204 temporarily retaining the part of micro-sample 206 is irradiated with an FIB 201 so as to be removed by sputtering, so that the part of micro-sample 206 is released from the retained state (FIG. 2(e)). As a result, a micro-sample 209 is completely separated and extracted from the sample substrate (FIG. 2(f)). Then, the micro-sample 209 separated and extracted from the sample substrate is moved to a position over a micro-sample holder 210 while being fixedly connected to the tip portion of the probe 207. When the micro-sample holder 210 enters a scan range of the FIB 201 by movement of the specimen stage, the movement of the specimen stage is stopped at that position, then the probe 207 is pushed downward to cause the micro-sample 209 to approach an upper surface of the micro-sample holder 210 (FIG. 2(g)). When the micro-sample 209 contacts with the upper surface of the micro-sample holder 210, an FIB 201 is irradiated onto a contact portion of them while introducing deposition gas, thereby to form a deposition film 211, so that the micro-sample 209 is fixedly connected onto the micro-sample holder 210 via the deposition film 211. The formed deposition film 211 is adhered at its one part onto the micro-sample holder 210 and at its another part to a side surface of the micro-sample 209, thereby to fixedly connect therebetween (FIG. 2(h)). Then, after stopping the supply of the foregoing deposition gas, the probe 207 is separated from the micro-sample 209 by irradiating an FIB onto the deposition film 208 fixedly connecting the probe 207 and the micro-sample 209 to each other so as to remove the deposition film 208 by sputtering, or by cutting the probe. As a result, the micro-sample 209 is fixedly retained on the micro-sample holder 210 and becomes completely independent of the probe 207 (FIG. 2(i)). Finally, the micro-sample is finished by FIB irradiation so that an observation desired area of the micro-sample becomes a membrane 212 having a thickness of about 100 nm or less, and a series of the TEM sample preparing steps is completed (FIG. 2(j)). Conventionally, a TEM sample was prepared through steps like the foregoing steps.
In the foregoing known example 1, it is possible to prepare a TEM sample for about one to two hours. However, in production of semiconductor devices, inasmuch as improvement in yield leads directly to improvement in profit, a failure analysis in a shorter TAT (Turn Around Time) is desirable. Therefore, it has been desired to further shorten a time required for preparing the TEM sample. Among the foregoing sample preparing steps, the step of fixing the micro-sample onto the micro-sample holder using the FIB assisted deposition requires about 15 minutes for the formation of the deposition film. On the other hand, although not relating to the TEM sample preparation, JP-A-9-85437 (known example 2) describes a method of using arc discharge as a method that can instantaneously fix a sample onto a substrate without using deposition. This method will be explained using FIGS. 3A to 3E. A micro-probe 301 is moved to a position over a metal particle 302 to be an object. At this time, a voltage applied across the micro-probe 301 and a conductive substrate 304 by a power source for high voltage direct current 303 is 0V (FIG. 3A). Then, the micro-probe 301 is brought into contact with the metal particle 302, and a voltage of about several tens of volts is applied. This applied voltage generates a static electrical force so that the metal particle 302 is adsorbed to the tip of the micro-probe 301 (FIG. 3B). Then, the micro-probe 301 raises the metal particle 302 and moves to a position over a predetermined position of the conductive substrate 304 (FIG. 3C). Then, the metal particle 302 is brought into contact with the predetermined position of the conductive substrate 304 (FIG. 3D) and, by applying a high voltage of about 10 kV in this state, the metal particle 302 is joined to the conductive substrate 304 by the use of contact arc discharge 305 generated between the conductive substrate 304 and the metal particle 302 (FIG. 3E). In this method, since the arc discharge is used, the metal particle 302 can be instantaneously joined to the conductive substrate 304. However, if this method is applied to the TEM observation sample preparation, taking into consideration that the high voltage as large as 10 kV is applied to the micro-sample being an observation object, that the micro-sample itself is melted due to arc welding, and that current flows through the micro-sample, the possibility can not be denied that the observation object to be subjected to a failure analysis has been changed in quality upon joining. Therefore, a fixing method that does not cause the quality change of the observation object is desirable.
Further, in case of fixing the micro-sample itself onto the sample holder as in the foregoing known example 1 or 2, it is necessary to ensure a height of the micro-sample for the following reasons. First, in case of the TEM sample of the known example 1, the TEM sample and its surroundings upon TEM observation are as shown in FIG. 4. Specifically, the micro-sample 209 is fixed on an end surface of the micro-sample holder 210, and an internal structure of the micro-sample 209 is observed by irradiating an electron beam onto the micro-sample 209 and transmitting it therethrough as shown by an arrow 401. FIG. 5A is a sectional view thereof at a position where the electron beam 401 passes, seen in a direction of an arrow 402. Herein, the membrane 212 is an area to be observed. Although the surface of the micro-sample holder 210 for fixing thereon the micro-sample 209 should be as flat as possible, it still has some roughness. As a result, as shown in FIG. 5B, if the height of the micro-sample is low, there occurs such an instance where the electron beam 401 transmitted through an observation area 501 is blocked by the micro-sample holder to disable observation.
On the other hand, in the TEM observation, it is generally performed to change contrast of an image depending on a structure or facilitate observation of a lattice image by matching a crystal orientation of a sample with an electron beam incident direction or intentionally deviating it. Thus, there is a case where the micro-sample holder 210 is inclined relative to the electron beam incident direction (direction of the arrow 401) as shown in FIG. 5C. In this event, if the height of a micro-sample 502 is small, there arises such an instance where the electron beam 401 transmitted through the observation area 501 is blocked by the micro-sample holder to disable observation. This inclined angle is, in general, set to about ±15 degrees or less. Taking into consideration facility of handling in terms of strength and the like, it is desirable that the micro-sample holder 210 has a thickness of about 30 μm. In this case, when, for example, the micro-sample 502 is placed at the center in a thickness direction of the micro-sample holder 210 as shown in FIG. 5C, an area in which the electron beam is not blocked by the micro-sample holder 210 even if the micro-sample holder 210 is inclined by 15 degrees, is an area above about 4 μm. Therefore, the height of at least 4 μm or more is required under the observation area 501.
On the other hand, there are those instances where an energy dispersive X-ray spectroscopic analyzing method (EDX) is used for conducting an element analysis of a sample using a TEM. This is a method wherein, as shown in FIG. 5D, when the electron beam is transmitted through the observation area 501 along the path of the arrow 401, an X-ray 503 is generated corresponding to an atomic species due to interaction with atoms inside the sample, and an element of the sample is identified by detecting such an X-ray using an X-ray detector 504. In this case, if the height of the micro-sample 502 is small so that the micro-sample holder 210 exists just near the observation area 501 as shown in FIG. 5D, there arises a problem that a scattered electron 505 scattered by the observation sample is also irradiated onto the micro-sample holder 210 to generate an X-ray 506 from the micro-sample holder 210, so that a constituent substance of the micro-sample holder 210 is detected as a spectrum by the X-ray detector 504, thereby causing background noise.
Taking into consideration the foregoing cases of the inclined observation and the EDX, it is desired that the observation area 501 of the micro-sample 209 is spaced apart from the micro-sample holder 210 as much as possible. As a practical size, the height of the micro-sample 209 is preferably about 10 μm to 15 μm. When preparing the micro-sample 209 of this size, it is estimated that the volume processed by the FIB, including the rectangular holes 202, 202′, the vertical trench 203, and the inclined trench 205 processed in FIGS. 2(b) and 2(c), is about 5000 μm3. This requires a processing time of about 30 minutes when an FIB of 30 keV and 10 nA is used for processing, and when a specimen is an Si device. Naturally, if a sharper beam, for example, an FIB with a reduced beam current of 5 nA, is used, the processing time will be twice, i.e. about an hour. For shortening the sample preparing time, the processing volume should be reduced. However, as long as the lower side of the micro-sample 209 is fixed onto the micro-sample holder 210, reduction in height of the micro-sample induces the foregoing problems. Thus, it has been difficult to shorten the sample preparing time.